In electrophotographic processes, a light image of an original document is typically recorded in the form of a latent electrostatic image upon a photosensitive member. The latent image is subsequently developed on the photosensitive member by applying electroscopic marking particles, commonly referred to as toner. The visual toner image is then typically transferred from the photosensitive member to another support member, such as a sheet of plain paper. The transferred image is then affixed to the support member typically, for example, by using heat and pressure applied at a fusing station.
In order to affix or fuse electrostatic toner material onto a support member by heat and pressure, the temperature of the toner material is elevated to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. This action causes the toner to flow to some extent into the fibers or pores of a support member. Thereafter, as the toner material cools, it solidifies which causes the toner material to be bonded firmly to the support member. In the electrophotographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support member has been to pass the support member bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls so the toner image contacting the fuser roll is heated within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. As long as the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the “hot offset” temperature of the toner, the toner particles in the image areas of the toner do not liquefy and cause a splitting action in the molten toner. This type of splitting is sometimes known as “hot offset.” Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces holding toner to a contacting surface, such as a fuser roll.
Occasionally, however, toner particles are offset onto the fuser roll by an insufficient application of heat to the surface of the fuser roll. This phenomenon, sometimes called “cold” offsetting; may be caused by imperfections in the properties of the surface of the roll or by weak electrostatic forces that are used to adhere the toner particles to the copy sheet. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to a backup roll in the fusing station during periods of time when no copy paper is in the nip. Toner particles may also be picked up by the fuser roll and/or backup roll during fusing of duplex copies or from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as “offsetting,” has been to provide a Viton fuser roll to which a release agent such as silicone oil is applied. The thickness of the Viton covering on the fuser roll is on the order of several mils and the thickness of the oil is less than 1 micron. Silicone based (polydimethylsiloxane) oils, which possess a relatively low surface energy, have been found to be materials that are suitable for use with heated Viton fuser rolls. In practice, a thin layer of silicone oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support member. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
One method for facilitating the release of a support member from the fuser roll is to apply a release agent to the fuser roll to reduce the degree of support member adhesion to the fuser roll to a point where the support member separates from the fuser roll as it leaves the nip at the fuser roll. One release agent material used to facilitate release of a support member is silicone oil. The silicone oil is applied to the fuser roller by a Release Agent Management (RAM) system. The RAM system includes a supply of release agent, a metering roll to transfer the release agent from the supply to the fuser roll. In some RAM systems, a donor roll may also be included to transfer the release agent from the metering roll to the fuser roll.
An example of a RAM system is disclosed in U.S. Pat. No. 4,214,549 issued on Jul. 29, 1980 to Rabin Moser. This patent illustrates a heat and pressure roll fusing apparatus for fixing toner images to copy substrates. The apparatus includes an internally heated, fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The pressure roll is the softer of the two rolls, therefore, the nip is formed by the harder fuser roll indenting the softer pressure roll. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a very thin silicon rubber or Viton material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump from which it is dispensed by means of a metering roll and a donor roll, the former of which contacts the release fluid in the sump and the latter of which contacts the surface of the heated fuser roll. The donor roll is provided with an outer elastomeric layer which is deformable by both the heated fuser roll and the metering roll. Thus, the driven fuser roll imparts rotational movement to the donor roll which, in turn, causes the metering roll to rotate. While a donor roll RAM system provides a microscopic, uniform layer of release agent on a copy, the donor roll also contributes a thermal load on the fuser roll during machine standby as well as during a printing run. Additionally, the release agent may also provide a thermal load on the fuser roll.
To reduce the thermal load imposed by the release agent, RAM systems have been modified to heat the release agent supply so the release agent applied by the metering roll to the donor roll is at a temperature higher than room temperature. In donor roll RAM systems, however, the relatively small amount of release agent may sufficiently cool before being applied to the fuser roll that the thermal load of the donor roll may affect the effectiveness of the fusing station. Other RAM systems have been modified to heat the metering roll. Because the metering roll is interposed between the release agent supply and the donor roll, both components are warmed by the heated metering roll and the thermal load of the donor roll on the fuser roll is reduced.
In RAM systems in which the metering roll is heated, the metering roll is open at one end so a heater may be inserted in the internal volume of the hollow metering roll. The metering roll is cantilevered mounted so the open end remains above the release agent level in the supply and the other end is coupled to a rotational driver. As the system is used, however, the cantilevered metering roll sags and the release agent is able to enter the internal volume of the metering roll. The direct heat of the heater on the release agent in the interior of the metering roll causes the release agent to decompose and form a residue. This residue, in turn, may flow back into the release agent supply and contaminate the release agent in the supply. Additionally, the internal heater is thermally coupled to the surface of the metering roll through convection and the distribution of heat along the metering roll may be very uneven. At start up, thermal gradients of as much as 100° F. between the center of the roll and its ends may be experienced. Furthermore, the cartridge heaters used for the metering roll internal heaters are relatively expensive.